Submitted to: Multicrop Aflatoxin and Fumonisin Elimination and Fungal Genomics Workshop-The Peanut Foundation
Publication Type: Abstract Only
Publication Acceptance Date: 9/1/2004
Publication Date: 10/25/2004
Citation: Cary, J.W., Harris-Coward, P.Y., Molyneux, R.J., Mahoney, N.E. 2004. Inhibition of aflatoxin biosynthesis by tannic acid [abstract]. Multicrop Aflatoxin and Fumonisin Elimination and Fungal Genomics Workshop-The Peanut Foundation. October 25-28, 2004, Sacramento, California. p. 25.
Technical Abstract: Tree nut research has demonstrated that aflatoxin (AF) biosynthesis appears to be inhibited by gallic acid (GA). It is hypothesized that the release of GA from hyrolyzable tannins present in the plant seed coat or hull tissue by fungal tannase enzyme is responsible for the observed inhibition of AF production. Studies have shown that GA content of the seed coat or hull correlated inversely with the ability of the fungus to produce AF. We have shown that inhibition of AF production appears to be at the level of transcription of AF biosynthetic genes as GA at 0.25 percent was shown to almost completely inhibit expression of nor1, ver1, and omtA genes, while aflR expression was only slightly reduced. In order to test the theory that GA is the major inhibitory compound in the seed coat/hull tissues, we have cloned the tannase gene from A. flavus and constructed a disrupted version of it by insertion of the A. parasiticus nitrate reductase (niaD) gene. We hypothesized that inhibition of toxin production should not be as great in the tannase knockout mutant due to its inability to release GA from tannic acid (TA). The disruption construct was transformed into an A. flavus 70 niaD mutant and two isolates were identified, 13B and 20A, that exhibited loss of tannase activity. Both the wild-type A. flavus 70 and the tannase knockout mutant 13B were grown for 3 days on minimal salts/1 percent sucrose plates supplemented with 0, 0.05, 0.125, or 0.25 percent GA or TA. Essentially no difference in radial growth was observed when comparing the wild-type to 13B on media supplemented with GA. No significant differences were observed between the two fungi for growth on TA either. However, significant differences were observed when comparing fungal growth on GA vs. TA. Radial growth of both isolates was the same at 0 and 0.05 percent GA or TA but reduced by 13.6 percent and 37.5 percent on 0.125 and 0.25 percent GA respectively, compared to TA. In addition, there was a reduction in sporulation of about 50 percent observed on media containing 0.125 and 0.25 percent GA vs. TA. Analyses of growth, aflatoxin production, and transcription of aflatoxin pathway and developmental genes during growth of the wild-type and 13B in shake culture in minimal salts/1 percent sucrose supplemented with TA will also be presented.